A Discrete Fourier Transform (DFT)-spread orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) system is particularly attractive for use in the uplink channel from a subscriber station to the base station. This is because a DFT-spread OFDM system has low peak-to-average power ratio (PAPR) characteristics. A key design parameter for OFDM and OFDMA power amplifiers is the requirement to minimize the generation of adjacent channel power (ACP) noise—undesired spectral components in adjacent transmission channels. Power amplifiers are more linear at lower input signal levels. Large input signals tend to drive a power amplifier into saturation. Thus, smaller signals experience less distortion and have lower ACP noise.
However, a sudden large signal peak still results in distortion and ACP noise. This is especially problematic in systems that have large peak-to-average power ratios (PAPRs), such as OFDM and OFDMA systems (i.e., multicarrier systems). To avoid this, power amplifiers often operate in back-off (BO) mode (i.e., reduced input signal) in order to accommodate large signal peaks. DFT-spread OFDM/OFDMA minimizes the PAPR in the uplink, which allows the subscriber station amplifier to operate with a smaller amount of back-off and provides the subscriber stations with higher transmit power and data rate.
Conventional DFT-spread OFDM/OFDMA systems may also use spectral shaping filters to reduce PAPR. However, the modulation type used by a subscriber station to transmit in the uplink is selected without taking into consideration the power efficiency resulting from signal spectral shaping. This results in poor selection of modulation type, thereby degrading the overall system throughout and performance.
Therefore, there is a need for improved multi-carrier (e.g., OFDM, OFDMA) transmission systems that minimize amplifier peak-to-average power ratio (PAPR) without suffering performance degradation. In particular, there is a need to maximize the performance of DFT-spread OFDM transmission techniques.